Process for electrolytic reduction



Patented Apr. 28,1942

UNITED STATES'PATENT OFFICE Atlas Powder Company, W corporationofDelaware n, Del., a

No Drawing. Application September 16, 1939, Serial No. 295,244

11 Claims. (Cl. 204-77 alcathode, and is separated from the anolyte.

comprising an aqueous solution of sulphuric acid, sodium sulphate orother suitable electrolyte, by a porous diaphragm. The anode in contactwith the anolyte, may consist of chemical lead or lead coated with leaddioxide. when electric current is passed through the cell, sulphate ionsare, discharged at the anode and subsequently react with the waterforming oxygen and sulphuric acid adjacentthe anode. In the catholyticsection of the cell, sodium atoms migrate to the cathode and unite withth mercury, forming sodium amalgam; which subsequently decomposes thewater, forming nascent hydrogen and sodium hydroxide. The nascenthydrogen formed in this way reduces the monosaccharide.

The reduction of solutions of sugars or composite sugar-bearing mixtureswhich have beenrendered capable of conducting electric current inaccordance with the foregoing, has been attended with low rates ofreduction, varying unaccountably at times to extremely low rates ofreduction. I

An object of the invention is to provide an improved process for theelectrolytic reduction of monosaccharides.

Another object of the invention is to provide greater efllciency andhigher and more uniform rates of reduction in the lectrolytic reductionof monosaccharides. Other objects will hereinafter I more fully appear.

In practising the process of reducing monosaccharides certain variationsof the process described in the said Creighton Patents have beenintroduced. It has been customary to use a catholyte comprisingan'aqueous solution of one or more monosaccharides containing as anelectrolyte an alkali metal salt, usual sodium sulphate. From thestandpoint oftavailability as 1 well as satisfactory performance, sodiumsulphate' is preferred although other alkali metal salts which do notcontain objectionable anions can be used. The alkaline earth metal saltshave not been as readily available and have other ob- :lectionablefeatures, e. g. the insolubility of some of their salts. For an alkalinereduction the catholyte has usually added to it also a small amount ofalkali such as sodium hydroxide. It is also possible toreducemonosaccharides while maintaining the catholyte in an acid conditioninstead of in alkaline condition. As a cathode there is used either anamalgamated metal plate, such as amalgamated lead or amalgamated zinc,or an unamalgamated zinc plate.

According to the present invention it has been discovered that thepresence of magnesium, even in very small amounts, exerts a surprisinghindering efiect on the reducing process. Particularly the presence ofmore than about 1.0 parts per million magnesium based on the totalsolids in the catholyte causes a very sharp drop in the efilciency ofthe reducing process. This hinder-1 ing eilect has been notedparticularly in the case of the reduction of glucose and inverted canesugar. The hindering eiIect is independent of the nature of the cathodeand has been observed both with amalgamated and unamalgamated cathodes.Furthermore, the deleterious action is not limited to alkalinecatholytes but is also noted in the case where the catholyte ismaintained in an acid condition.

reduction where the catholyte contains about 13.0 parts per millionmagnesium. Again, where the magnesium content is further cut down to notmore than about 1.6 parts per million a further improvement [in the rateof reduction is noted. Below this concentration of magnesium the rate ofreduction is approximately constant.

In the case of the reduction of inverted cane sugar and the othermonosaccharides, the same general phenomenon appears. a

Although magnesium as such is not added to the catholyte before orduring the reduction, it was discovered that such small amounts as thosefound-harmful were frequently introduced as impurities in either theoriginal ingredients of,

the catholyte or-materials added from time to time during the reduction.For example, the sodium sulphate or other alkali metal salt used as theelectrolyte in the catholyte frequently contains small amounts ofmagnesium salts as impurities. The sugars themselves may be contaminatedwith small quantities of magnesium. The water used in making up thecatholyte frequently contains magnesium, particularly where it has notbeen purified by distillation prior to its use. It is the usual practiceduring the course of the reduction to control the pH of the catholyte bythe addition of alkali or acid and the magnesium content was found to beincreased frequently due to the presence of this element as an impurityin the base or acid employed. A.

further source of magnesium was found to be the porous diaphragm orother parts of the cell in which the reduction was conducted.

It is necessary therefore that the materials making up the catholyte andanolyte as well as albparts of the electrolytic cell which may come incontact with the catholyte during the reduction process, be suflicientlyfree of magnesium so. that the catholyte does not become contaminated tothe extent which has been found harmful.

In the table, examples are given showing the increase in timereciuiredfor reduction of 90% of the sugar at amalgamated lead cathodeswith increased contamination of the catholyte with magnesium. In theexamples of the table, the initial composition of. theagueous catholytein grams p r liter was:

Glucose v 325 NarSOe 75 NaOH 10 The glucose was reduced to a mixture ofsorbitol and mannitol. In each run the current density was reduced from1.07 to 0.54 amp./dm. at the time approximately 65% of the sugar wasreduced. The catholyte temperature was maintained throughout thereduction at about 68 F.

, It can be seen from the foregoing table that when the catholytecontains more than about 7.0 parts per million magnesium on the basis oftotal solids, a decided slowing up of the rate of reduction results.Thus, in Example 8 a magnesium content of 7.19 parts per million alloweda current efficiency of 37.0% to be obtained in the reduction, whereasin Example 9, 13.56 parts per million magnesium lowered the currentefliciency to 25.5%. Example 8 was therefore nearly 50% better thanExample 9 as to current eiliciency. In Example 7 /4.0 parts per millionmagnesium allowed a current efliciency of 49.6% to be obtained, which isnearly double the efiiciency of Example 9. A further improvement incurrent efliciency is noted in Example wherein the magnesium content was1.56 parts per million.

was no substantial improvement in current efliclency.

According to the invention, therefore, the magnesium content is limitedto not more than about 7.0 parts per million. More specifically, 4.0parts per million shows very high efllciency and for the best currentefficiency not more than about 1.6 parts per million magnesium should beplaced as the tolerance.

By placing a tolerance of 1 part per million maximum magnesium contenton the ingredients such as sugar, caustic soda, sodium sulphate, and 0.3part per million maximum magnesium content on sulphuric acid employed inthe control of the pH of the catholyte and in making up the 'anolyte,and further by selecting material for cell construction which does notcontaminate the catholyte with magnesium whereby the catholyte ismaintained throughout the reduction free from magnesium in an amountgreater than 1.6 parts per million of total solids, excellent rate ofreduction have been obtained.

Although in the table the advantages of the present invention have beenpointed out particularly with respect to the reduction of glucose, it

is to be understood that it applies also to the reduction ofmonosaccharides generally, such as invert sugar, fructose, mannose,galactose, sorbose, xylose, etc.

The present application is a continuation in part of my application Ser.No. 180,533.

In the following claims where I have used the term alkali metal salt Imean to include all of the alkali metal salts which do not haveobjectionable anions. For example, the sulphates are satisfactory while,for example, chlorides and nitrates have too corrosive an effect on theanodes for practical use and may be considered objectionable.Objectionable anions" therefore refers to any anion which interfereswith the reduction of the sugar or which attacks the anode to anundesirable extent.

The invention has been described with reference to the examples but theinvention is to be taken as limited only by the scope of the followingclaims.

I claim: 1. The process for the preparation of polyhydric alcohols whichcomprises electrolytically reducing an'aqueous monosaccharide solutioncon magnesium in amount greater than about 7.0

parts per million based on total solids during the reduction.

3. The process for the preparation of polyhydric alcohols whichcomprises electrolytically reducing an aqueous monosaccharide solutioncontaming an alkali metal sulfate as a catholyte in a diaphragm cell,and maintaining said catholyte. free from magnesium in amount greaterthan about 7.0 parts per million based on total solids 1 during thereduction.

Below this value-of 1.56 parts per million there t. The process for thepreparation of hexahydric alcohols which comprises electrolyticallyreducing an aqueous glucose solution containing an alkali metal sulfateas the catholyte in a diaphragm cell, and maintaining said catholytefree from magnesium in amount greater than about 7.0 parts per millionbased on total solids during the reduction.

5. The process for the preparation of polyhydric alcohols whichcomprises electrolytically reducing an aqueous monosaccharide solutioncontaning an alkali metal sulfate as the catholyte in a diaphragm cellhaving an amalgamated metal cathode, and maintaining said solution freefrom magnesium in amount greater than about 7.0 parts per million basedon total solids during the reduction.

6. The process for the preparation of polyhydric alcohols whichcomprises electrolytically reducing an aqueous monosaccharide solutioncontaining an alkali metal salt as the catholyte in the diaphragm cell,and maintaining said catholyte free from magnesium in amount greaterthan about 4.0 parts per million based on total solids during thereduction.

7. The process forthe preparation of polyhydric alcohols which compriseselectrolytically reducing an aqueous monosaccharide solution containingan alkali metal salt as the catholyte in the diaphragm cell, andmaintaining said catholyte free from magnesium in amount greater thanabout 1.6 parts per million based on total solids during the reduction.

8. The process for the preparation of hexahydric alcohols whichcomprises electrolytically reducing an aqueous glucose solutioncontaining sodium sulfate as the catholyte in a diaphragm cell, andmaintaining said solution free from magnesium in amount greater thanabout 4.0 parts per million based on total solids during the reduction.

9. The process for the preparation of hexahydric alcohols whichcomprises electrolytically reducing an aqueous glucose solutioncontaining sodium sulfate as the catholyte in a diaphragm cell, andmaintaining said solution free from magnesium in amount greater thanabout 1.6

parts per million based on total solids during the reduction.

10. The process for the preparation of hexahydric alcohols whichcomprises electrolytically reducing an aqueous glucose solutioncontaining sodium sulfate as the catholyte in a diaphragm cell having anamalgamated lead cathode, and maintaining said solution free frommagnesium in amount greater than about 4.0 parts per million based ontotal solids during the reduction.

11. The process for the preparation of hexahydri alcohols whichcomprises electrolytically reducing an aqueous glucose solutioncontaining sodium sulfate as the catholyte in a diaphragm cell having anamalgamated lead cathode, and maintaining said solution free frommagnesium in amount greater than about 1.6 parts per million based ontotal solids during the reduction.

' KENNETH R. BROWN.

